It is known in the art that heat-stable salts (HSS) are formed during the absorption of SO.sub.2 in alkanolamine solvents. For example, the sulfate concentration in the absorbent build-up both by absorption of sulfur trioxide and/or sulfuric acid mist which might be present in the stack gas; and by oxidation of dissolved sulfur dioxide with oxygen. A further source of the buildup of sulfates or other sulfur oxyanions is by disproportionation of sulfites and bisulfites. Such HSS include, in addition to sulfates, SO.sub.4 =; thiosulfates S.sub.2 O.sub.3 =; dithionates S.sub.2 O.sub.6 =; trithionates S.sub.3 O.sub.6 =; higher polythionates, S.sub.x O.sub.6 =; and other sulfur oxyanion-containing HSS.
A number of methods are known for the removal of HSS from the solvent. For example, the sulfates usually can be removed essentially quantitatively through the use of an alkali metal hydroxide equivalent to twice the molar concentration of the sulfate. This results in substantially quantitative precipitation of the sulfate as the di-alkali metal salt without precipitation of sulfite or bisulfite ions (U.S. Pat. No. 3,872,212). However, other divalent sulfur oxyanions such as thiosulfates, dithionates and higher polythionates also build up in the system and cannot be quantitatively removed by means of alkali metal hydroxide precipitation. Furthermore, the presence of such other divalent sulfur oxyanions of HSS actively interfere with the quantitative removal of the sulfates.
In some instances, as in U.S. Pat. No. 3,503,185 the combustion waste gas is prewashed to remove sulfates which are then purged from the system. Such prewashes are not capable of removing all sulfur trioxide as sulfate and, of course, would not remove sulfates formed in other parts of the process. This patent, furthermore, does not disclose any means for eliminating the thiosulfates, dithionates and higher polythionates.
U.S. Pat. No. 3,790,660 is similar to the above, in showing a water prewash to remove sulfur trioxide and fly ash. In addition, it specifies a purge stream to remove the sulfate as alkali metal salt. There is no system disclosed, however, for removing the thiosulfates, dithionates or other polythionates except by purging them with the sulfate in a waste stream. The latter is relatively dilute, poses a pollution problem in disposal and is expensive to handle.
The use of alkanolamines, such as triethanolamine (TEA), has been found to be a highly efficient way of absorbing SO.sub.2 from waste gases in a cycle in which the alkanolamine solvent contacts the effluent stream to absorb the sulfur oxides, and is thereafter stripped by heat to release SO.sub.2, whereupon it is collected for safe disposal. The stripped alkanolamine is then recycled back to the absorber for further contact with incoming waste gases and further absorption of sulfur oxides. This type of system is disclosed in U.S. Pat. Nos. 3,620,674 and 3,904,735. HSS such as those mentioned herein above, accumulate in the recycling solvent to a troublesome extent and must be removed in order to maintain the absorption capacity of the solvent. The latter patent does disclose a sulfate purge cycle in which a portion of the lean absorbent is treated with potassium hydroxide or carbonate to precipitate out the sulfate as potassium sulfate. While this type of purge system is quite effective in removing sulfates, it is severely limited in removing other divalent sulfur oxyanions, which also interfere with the sulfate removal. There is no provision made in U.S. Pat. No. 3,620,674 for removing these HSS from the absorbent which gradually but inevitably loses effectiveness because of the accumulation of HSS therein.
Anion exchange resins have also been used to treat the total amount of a recycling absorbent, as in U.S. Pat. Nos. 3,896,214 and 3,833,710. Both patents are based on removing all the SO.sub.2 and SO.sub.3 from the waste gases, by utilizing ion exchange. This requires the utilization of extremely large amounts of resin and the use of large quantities of reagents to regenerate the resin. This is not only expensive but presents a considerable liquid waste disposal problem. In U.S. Pat. Nos. 4,122,149 and 4,170,628, the use of anion exchange resins is disclosed for the removal of heat stable sulfur oxyanions. In this case, the solvent is aqueous alkanolamine which is used in a cyclic process to separate SO.sub.2 from waste gas mixtures, and only a slip stream of the recirculating solvent is passed through the ion exchange resin. However, the chemical and resin costs involved in this step comprised a significant fraction of the operating costs of the overall process.